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Changing Fish Fauna of the Southwest 379 r ;i ;i specimens of Cyprinodon macularius there on September 20, 1927 (CAS 18561). In July 1928, C. E. Burt sampled the fishes at the same place and obtained all but C. insignis and C. macularius plus an ad- ditional species, the speckled dace. In September 1938, Carl L. Hubbs secured the same species taken by Burt. In April 1959, our party found the stream almost fishless, and took only Agosia. The creek exhibited obvious signs of severe erosion, and the area between Pata- gonia and Nogales showed clearly the effects of serious overgrazing, with impoverished and badly eroding soil. Many of the great cotton- woods along Sonoita Creek, and in the Santa Cruz River valley, above and below the mouth of that creek, had died-probably as a re- sult of the drastic lowering of the water table that has been acceler- ated since 1950 by the swelling population in southern Arizona. The original stands of tall grass had long since been replaced by mesquite, cholla, prickly pear, and burroweed. For many years the Santa Cruz River, intermittent from near Nogales almost to Tucson, rose to the surface shortly above San. Xavier Mission, about 8 miles south of Tucson. Here, on March 29, 1904, Chamberlain obtained 5 species: Agosia chrysogaster, Gila ro- busta intermedia, Catostomus insignis, Pantosteus clarki, and Poecil- iopsis occidentalis. By April 25, 1937, when Allan R. Phillips sampled this perennial flow, only the resistant Agosia remained, and this is the only species that I found there on July 12, 1939. By April 13, 1950, the flow had disappeared, and I was informed by Raymond Hock (then of the University of Arizona) that it went dry for the first time during the previous winter. Even in early historic time, the Santa Cruz ordinarily had no surface flour from some distance below Tucson to its confluence with Gila River. It formerly maintained a permanent flow in the headwaters, near Loehiel (Schwalen, 1942, pl. II), but pumping in the San Rafael Valley eliminated this surface water and its fishes (Gila, Agosia, Pocci,liopsis) between 1950 and 1956 (C. H. Lowe, Jr., personal communication). EXTINCT AND ENDANGERED SPECIES Since the close of the nineteenth century, no fewer than 6 species, and probably 7, have become extinct in the American Southwest. At least 13 additional forms have either been locally exterminated or are threatened with depletion to levels from which they might not be able to recover. In the following discussion, extinct species are treated 41 r 11